Conventionally, an erbium-doped fiber amplifier (EDFA) and a semiconductor optical amplifier (SOA) are used for amplifying a light signal. Actually, however, for eliminating noise generated in such an amplifier, the light signal that is transmitted at a high speed is once converted into an electrical signal, so as to be subjected to noise elimination and signal processing in an electronic circuitry, and the processed signal is then reconverted into a light signal to be transmitted. This incapability to achieve direct processing of a light signal without its conversion into an electrical signal limits the speed of the optical signal processing.
Therefore, in broad-band information services such as dynamic image communication and picture image distribution using fiber-optic communication capable of achieving broad-band high-speed data transmission, namely, in a field in which a large amount of information is to be processed at a high speed, there has been demanded a technique which enables a light signal to be processed without its conversion into an electrical signal. However, in a filed of optoelectronics, there have not yet realized high-performance signal amplifiers corresponding to a negative-feedback amplifier and an operational amplifier which are known in a field of electronics. That is, there is a strong demand for an advanced optical signal amplification device capable of achieving various kinds of signal processing without generating large noise.
The present inventor has proposed an optical-signal amplification triode device equipped with a function of providing an output signal light which is obtained by amplifying signal of an input signal light with use of a control light, as described in Document 1 or Document 2. The Document 1 discloses an optical control device, in which a first input light having a first wavelength, together with a second input light (bias light) having a second wavelength and a constant intensity, is inputted into a first optical amplifying element, and then an output light outputted from the first optical amplifying element, together with a control light having the first wavelength, is inputted into a second optical amplifying element, whereby an output light having been subjected to a switching control and other processing is obtained from the second optical amplifying element. Either of the first and second optical amplifying elements used in this optical control device utilizes cross gain modulation effect. The Document 2 discloses an optical function device, in which a first input light having a first wavelength, together with a second input light (bias light) having a second wavelength and a constant intensity, is inputted into a first optical amplifying element, and then an output light outputted from the first optical amplifying element, together with a control light having the first wavelength, is inputted into a second optical amplifying element. A part of an output light outputted from the second optical amplifying element is subjected to a positive feedback to an input side of the first optical amplifying element, whereby a greatly amplified output light is obtained at a high modulation factor.
[Document 1] WO 02/061502 A1
[Document 2] WO 02/086616 A1
In general, since a conventional optical amplifying element merely has a simple signal amplification function, the amplifying element disadvantageously amplifies not only the signal but also the noise. Therefore, gain, waveform and baseline of the output signal light are not stabilized in relation with the intensity and frequency of the input signal light, thereby making difficult to achieve an advanced signal processing. Further, in a case where an input signal light of an excessively high intensity is inputted to the conventional optical amplifying element, there is an inconvenience that the input signal light is further amplified and is outputted as surge light. Further, also in the above-described optical control device and optical function device, there is required an improvement such as further stabilization of the gain in the processing of the light signal.
The present invention was made in view of the above-described background circumstances. It is therefore an object of the invention to provide an advanced negative-feedback optical signal amplification device which is capable of providing an output signal light whose gain, waveform and baseline are stabilized without generating large noise, or whose intensity is suitably suppressed where an input signal light of an excessively high intensity is inputted to the optical signal amplification device.